The present invention relates to a relay apparatus for performing mutual conversion between IP (Internet Protocol) packet and ATM cell in a WAN (Wide Area Network) utilizing an ATM (Asynchronous Transfer Mode) technology.
Conventionally, a method has been known in which WAN is configured by connecting a plurality of LANs (Local Area Network) via an ATM network. For example, in an LAN environment, a protocol of TCP (Transmission Control Protocol)/IP is commonly used to transmit or receive the voice data or image data in the form of IP packet. Also, in the ATM network various kinds of data are transmitted or received in a unit of ATM cell. Therefore, a relay apparatus for performing mutual conversion between IP packet and ATM cell is necessary between the LAN and the ATM network. This relay apparatus performs conversion from the IP packet to the ATM cell, or from the ATM cell to the IP packet.
In the LAN environment utilizing the IP packet, the media communication which has strict limitations for the delay, such as the voice or image communication, assures the QoS (Quality of Service) in such a way that the amount of delay is minimized by setting at “1000” the value of a TOS (Type Of Service) field of 4 bits that is a service type included in an IP header placed at a leading part of an IP packet.
In the ATM environment, a service category can be specified for every virtual connection VC (Virtual Connection), and a fixed bandwidth can be secured by setting this service category at a CBR (Constant Bit Rate). Therefore, QoS with less delay time can be assured, irrespective of whether or not there is communication data.
In the asymmetric xDSLs, there is an Asymmetric DSL (ADSL) modem which is provided with the G.DMT standard having a downstream transmission speed of about 6 Mbits/sec and the G.lite standard having a downstream transmission speed of about 1.5 Mbits/sec. Both of the G.DMT and G.lite standards employ Discrete Multitone (DMT) modulation.
By the way, various kinds of data are transmitted or received using the IP packet in the LAN. In the case where the LAN and the ATM network are connected via a relay apparatus, the IP packet corresponding to media data having strict requirement for real time and the IP packet corresponding to burst data such as FTP (File Transfer Protocol) having less strict requirement for real time are transmitted or received via one IP connection, when an IP connection for transmission or reception of the IP packet and a virtual connection for transmission or reception of the ATM cell are matched. As a result, there was the problem that a suitable service quality cannot be attained to accommodate the virtual connection VC of the ATM network.
For example, in the case where only the IP packets for voice data having severe requirement for real time are transmitted or received on the IP connection, the CBR may be specified as a service category of the virtual connection for ATM. For the FTP data having less severe requirement for real time, a UBR (Unspecified Bit Rate) may be specified as a service category of the virtual connection for ATM. However, in the case where these two kinds of IP packets are mingled on one IP connection, it is impossible to make QoS control for ATM with the service category specified. Therefore, the IP connection and the ATM virtual connection corresponding to these two kinds of IP packets need to be prepared respectively, giving rise to the use of more resources, resulting in the higher operation costs.